The first months of 2018 have seen a flurry of research on human aging. Some news reports predict humans will soon be living much longer. But new science research indicates otherwise.
Birth and early childhood were a historically risky time. Tetanus, whooping cough and a range of infections took a severe toll on youngsters. Pioneer families had large numbers of children but few would survive and grow up to take care of their parents in old age. Today, half a century into the use of vaccines and antibiotics, couples average two children and they survive.
Average life expectancies have increased in the last century. A child born in America in 1900 would on average live to be 49. Today, average American life expectancy is up to 79. Worldwide, women tend to live longer and men live shorter lives. The current longest average life expectancy is 83 years in Japan.
But today’s increase in older populations is not an extension of maximum human longevity. Back in Greek and Roman days, a few people lived into their eighties and nineties. Today, many more folks are living to that age. But the maximum life expectancy has remained about the same.
In 1997, Jeanne Calment died in a nursing home in France at the age of 122, the oldest person with a confirmed birth date. However, the likely maximum old age for most people is 115, according to research by Vijg and colleagues (“Evidence for a limit to human lifespan” in the journal Nature 538: 257–259).
In the early 1800s, Benjamin Gompertz in Britain examined extensive death records and did the math. After age 30, a person’s risk of dying doubles about every 8 years. This formula, called the Gompertz law, applies to other adult mammals as well. For individuals who make it to 100 years, their chances of having a birthday at 101 drop to about 60 percent.
But then a strange turn occurs at age 105. In this June 28 issue of the journal Science, Italian researcher Elisabetta Barbi and colleagues examined an extensive Italian database and found that the risk of dying no longer increases after 105. They do not know the cause for this “risk plateau.”
Long ago, Leonard Hayflick had described our decline into old age as our inability to completely repair the cell damage we accumulate. So perhaps, at age 105, our body lives at a slower rate and produces less cell damage. This would then allow us to repair more of our cells, a theory that will require further research.
However, turtles—that are not active mammals and therefore live at a much slower rate—do live for much longer times without any reduction in physical abilities. This is where science makes a careful distinction. “Aging” is merely the passage of time. “Senescence” is a decrease in ability over time. Therefore turtles “age” without “senescence.”
Yet, even if we retained all of our youthful strength and vigor in old age, there would still be a limit on our lifespan. This is the “broken test tube hypothesis.” Glass test tubes do not weaken over time. But some can be broken each year, just as some living organisms die due to diseases or predators each year. So we can take a set of perhaps 10 test tubes and randomly break three each year. We then replace them with three new ones (reproduction). As years go by, although there is no difference in each test tube’s strength, this random breakage of test tubes will produce an average “age limit.” The chances of any test tube avoiding the random one-in-four breakage year-after-year will make it unlikely any test tube survives more than 12 years. This may explain why slow-living turtles eventually die.
But high-energy mammals such as humans must have been selected to have a limited life span.
A famous scientist once remarked that new ideas in science do not get accepted because of the force of their arguments, but because the old scientists with old ideas die off. Or to put it into an everyday setting, if you are over 60 and trying to work that new smartphone, you probably have to ask for help from a youngster. In a changing world, there is natural selection for re-starting the learning process. And whether we decline in ability with age, or are random broken test tubes, death is good for the species.
John Richard Schrock is a professor at Emporia State University.
